The application deals with on-going study of the differentiation of the small intestine from the embryonic to the mature state. Current work is being carried out on organ cultures of the small intestine of the chick embryo. The technique involves maintaining small fragments of duodenum or jejunum in a fluid medium of defined composition, without serum. Work is focused on the ultrastructure of the epithelium and its enzymic constitution. In the coming year we expect to continue work on the nature of the effect of thyroxine on alkaline phosphatase; on the reasons why sucrase-isomaltase differs from other alpha-disaccharidases in its responsiveness to hormones; and on the varying effects of actinomycin D according to concentration of the antibiotic and the time of application during culture. We also plan to investigate the extent to which duodenum and jejunum differ intrinsically, and to try to identify the time at which the difference between the two regions becomes fixed.